Many integrated circuits require a stable reference voltage and/or a stable reference current for operation. Examples of circuits which may use such references include data acquisition systems, voltage regulators, virtual grounds, measurement devices, analog-to-digital converters and digital-to-analog converters. The bandgap reference circuit is a common circuit solution for supplying either a voltage reference or current reference. Bandgap reference circuits are illustrated in U.S. Pat. Nos. 5,684,394, 5,349,286, 5,168,209, 4,939,442, 4,906,863 and 4,362,984, all assigned to Texas Instruments Incorporated.
FIG. 1 shows an exemplary prior art bandgap circuit 10. The circuit 10 shown in FIG. 1 is similar to the circuit disclosed in the aforementioned U.S. Pat. No. 5,349,286, especially FIG. 3 thereof. Bandgap circuit 10 is made of three parts, a startup circuit 12, a current reference circuit 14 and a voltage reference circuit 16.
The startup circuit 12 is of well known construction and operation and is not directly relevant to the present invention, so it is not described further herein. Construction and operation of the current reference circuit 14 is similar to that of the circuit of FIG. 3 of the aforementioned '286 patent. P-channel metal oxide semiconductor (PMOS) devices M1, M2, M3 and M4 are connected to a voltage supply, V.sub.cc, and function as a first MOS cascoded current mirror providing current to bipolar transistors Q1 and Q2, which are configured as a bipolar current mirror. One side of the first MOS current mirror, comprising devices M1 and M3, provides current to the collector of Q1. The other side of the first MOS current mirror, comprising devices M2 and M4, provides current to the collector of Q2.
Q1 and Q2 are sized differently; Q2 is typically four times as large as Q1 Therefore, although they conduct the same current, they have different current densities. As a consequence, there is a difference in their V.sub.be voltages and the difference is reflected in the current through resistor R1.
The output voltage V.sub.bg from voltage reference circuit 16 is a voltage that is a function of the current through resistor R2 and of the base emitter voltage, V.sub.be, of bipolar transistor Q3. Since the current through resistor R2 is mirrored from devices M2 and M4, it is seen that the current through PMOS devices M10 and M11 is a function of .DELTA.V.sub.be between bipolar transistors Q1 and Q2 and resistor R1. Therefore, V.sub.bg is a function of the .DELTA.V.sub.be between bipolar transistors Q1 and Q2, the ratio and resistor values R1 and R2, and the V.sub.be of bipolar transistor Q3.
An n-channel metal oxide semiconductor (NMOS) device M5 is configured as a "beta-helper". PMOS devices M6, M7, M8 and M9 are configured as a second MOS cascoded current mirror which forms a compensation circuit that measures the base drive of bipolar transistors Q1 and Q2 in the current generation circuit 14 and creates a supplemental current for resistor R2 and bipolar transistor Q3 in voltage generation circuit 16. Resistor R2 and bipolar transistor Q3 take the supplemental current and translate it into a supplemental voltage. The supplemental voltage cancels the error provided by current generation circuit 14 due to low gain bipolar transistors Q1 and Q2.
In high performance applications such as voltage regulators, the compensation methodology described hereinabove significantly reduces the error associated with finite gain bipolar transistors in voltage and current reference circuits. In fact, the circuit of FIG. 1 has proven to be an excellent bandgap voltage reference source. However, it has been discovered that certain shortcomings are encountered with the circuit of FIG. 1, especially when it is used in conjunction with newer technologies, which typically have lower Early voltages. Specifically, it has been discovered that variations in the supply voltage V.sub.cc cause corresponding variations in the output voltage V.sub.bg, thereby degrading the desired stable voltage performance. Even more problematically, as process dimensions diminish, it has been discovered that the bandgap voltage variation with power supply voltage variation is correspondingly greater as well.
It is an object of the present invention to provide a compensation apparatus that reduces the problems associated with power supply voltage variations in voltage and current reference circuits. It is another object of the present invention to provide such a compensation apparatus for bandgap reference circuits. Other objects and advantages of the invention will be come apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein.